The use of emulsions is widespread in the field of cosmetics. An emulsion is understood to mean a finely distributed mixture of two normally immiscible liquids without visible segregation.
In an emulsion there is a finely distributed mixture of two liquids present, such as oil and water. Here, one of the liquids (phase) forms small droplets which are present distributed in the other liquid (phase). The phase which forms the droplets is referred to as the inner phase or also the disperse phase. The phase in which the droplets are floating is called the outer phase or also the continuous phase.
Emulsions belong to the disperse systems and differ from mixtures of miscible liquids. Emulsions are generally cloudy, milky liquids. In the case of emulsions that comprise a water phase and an oil phase, a distinction is made between oil-in-water emulsions (O/W emulsions) and water-in-oil emulsions (W/O emulsions).
Emulsions are thermodynamically unstable. The disperse (inner) phase tends to want to amalgamate by coalescence to form larger areas—here, the interfacial energy between the two phases is reduced. Emulsions used in the field of cosmetics, however, should be maintained usually over a specific period of time (between a few hours and a few years) and under specific conditions (for example within specific temperature and pH value ranges).
In order to stabilize the emulsion, an emulsifier, i.e. a surfactant or a surface-active compound, is thus generally used, which facilitates the formation of the droplets and counteracts a segregation (phase separation). Emulsifiers lower the interfacial tension between the phases by forming interfacial films at the phase boundary between oil and water, whereby an irreversible flowing together of the droplets (coalescence) is counteracted. Cloudy emulsions having a droplet size in the micrometer range are generally produced.
An emulsion can also be stabilized by the addition of specific solids. Solid-stabilized emulsions are often called Pickering emulsions, after their discoverer. S. U. Pickering demonstrated in 1907 that small particles which are better wetted by water than by oil can stabilize O/W emulsions. What is important for sufficient stabilization is that a mechanically stable solids film can form around the dispersed phase.
A cosmetic Pickering emulsion can be, for example, an emulsion which is stabilized by colloidal silica particles. These silica particles arrange themselves at the interface between the two phases (hydrophilic and lipophilic phase) and prevent the droplets of the disperse inner phase form coalescing (amalgamation of the droplets).
Pickering emulsions or solid-stabilized emulsions include particulate solids for stabilizing the emulsion—said solids can be used either instead of surfactants or also in addition to surfactants. A key advantage of solid-stabilized emulsions lies in the fact that the surfactant concentration in the emulsion can be heavily reduced. After reduction of the surfactant concentration, solid-stabilized emulsions or Pickering emulsions are also generally characterized by a very good long-term stability.
A further advantage of Pickering emulsions lies in their greater stability with respect to changes of the chemical medium, for example a change to the pH values or the salt concentrations.
The principle of the stabilization of cosmetic emulsions by the use of particulate solids is already known from the prior art. By way of example, EP 0987002 describes Pickering emulsions which are characterized by an oil phase, a water phase, and at least one type of microfine particle having a mean particle size of less than 200 nm. The emulsions of EP 0987002 should be suitable for use as cleansing emulsion, as face or body care preparation, as sunscreen product, or as deodorant, and in particular should have an improved skin compatibility.
Fine-grain W/O emulsions comprising oil droplets in the micrometer range, which are free from surfactants and are stabilized only by solids, are described in EP 0686391. Here, spherical polyalkylsilsesquioxane particles are used having a diameter of from 100 nm to 20 μm. In EP 870495 fine-grain O/W emulsions comprising oil droplets in the micrometer range are described, wherein here surfactants as well are used as emulsifiers in addition to fine-grain solids having a diameter of up to 200 nm.
In U.S. Pat. No. 3,920,883 and U.S. Pat. No. 4,767,741 surfactant-free O/W macroemulsions in the form of droplets having a particle diameter of the oil droplets in the range of 0.1 millimeters to several centimeters are disclosed, in which case fine-grain solids particles are likewise used as emulsifier.
In all of the aforementioned documents, emulsions are described that are produced as a mono-component product, i.e. the emulsion in question—whether a skin cream, sunscreen product, cleanser, or deodorant—is applied directly after removal from the container in which it is provided.
The use of solid-stabilized emulsions has not previously been known in cosmetic products of which the use requires the prior mixing of two or more separately packaged preparations.
Hair dyes, in particular oxidative hair dyes, are a known example of cosmetic products in which the user, before said products can be used, must first produce a ready-to-use mixture by mixing various preparations.
Oxidative color-changing agents usually comprise a first component, which includes the oxidizing agent. This first component is mixed with a second, separately packaged component. This second component for example includes an alkalizing agent and/or oxidation dye precursors, or what are known as developer components and coupler components. The developer components form the actual dyes under the influence of oxidizing agents or of atmospheric oxygen, either amongst themselves or with coupling to one or more coupler components. In order to prevent a premature, undesirable reaction with one another, the oxidizing agent (hydrogen peroxide) and the oxidation dye precursors (or the alkalizing agent) are expediently only brought into contact with one another immediately before use.
In order to produce a ready-to-use oxidative dye, the user must therefore mix the first preparation, which includes the oxidizing agent, with the coloring substances (i.e. the oxidation dye precursors and/or the direct dyes).
Both the oxidizing agent preparation (A) and the dye (B) (which includes the oxidation dye precursors and/or the direct dyes) are generally emulsions. Here, the viscosity of each emulsion must be selected such that both emulsions on the one hand are thin enough to ensure complete and homogenous mixing, but on the other hand are also sufficiently thick to avoid a dripping of the finished ready-to-use mixture.
Both the oxidizing agent preparation (A) and the dye (B) are thus set optimally to a precisely defined viscosity range.
Particularly in the field of hairdressing, the oxidizing agent preparation (A) is often supplied to the hairdresser in a large bulk bundle. In order to ensure a comfortable and quick removal, the oxidizing agent preparation (A) is often set to a very low viscosity. Emulsions of low viscosity in particular often have serious weaknesses in respect of their storage stability.
It is therefore desirable to provide an oxidizing agent preparation for oxidatively changing the color of keratin fibers, in particular human hair, which preparation is present in the form of an emulsion, can be set to a low viscosity range, and yet still has a very high storage stability. These oxidizing agent preparations should be able to be mixed particularly easily and quickly with a second preparation (for example a coloring cream), such that a homogenous ready-to-use mixture in the optimally set viscosity range is produced after the mixing. In addition, the viscosities of the oxidizing agent preparation and of the finished ready-to-use mixture should lie in the desired specification range, even after a longer storage time, i.e. the viscosities of oxidizing agent preparation and ready-to-use mixture should not change or shift unpredictably even after storage of the oxidizing agent preparation over a number of weeks. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with this background of the invention.